1. Field of Invention
This invention relates to tunable laser systems and in particular to high power and highly agile directly driven grating tuned devices.
2. Prior Art
Laser radar (LIDAR) systems, utilizing tunable lasers, can be used to transmit different wavelengths of light into airborne suspensions (such as smog or poison gasses) which have differing reflectance""s or absorption to different wavelengths. The reflected light intensity is then measured for remote spectrographic analysis of suspension samples. It is advantageous to maximize the stability and repeatability of the output at each different wavelength. It is also advantageous to minimize intervals between transmitting wavelengths in order to reduce measurement interference by relative motion between the LIDAR unit, the intervening atmosphere and the suspension sample. Maximum accuracy is achieved by successively transmitting different wavelengths with constant power at the laser""s maximum cyclic rate.
Tunable lasers typically include an intra-cavity diffraction grating. The wavelength of such lasers is tuned by adjusting the angle of incidence of the laser cavity beam against the diffraction grating. Such intra-cavity tuning requires very high accuracy and stability. Tuned CO2 lasers, for instance, require a grating angular range of typically 0.2 radians and an accuracy of 10 or 20 xcexcradians. Output laser power is a sensitive function of the tuning angle near a particular wavelength.
Laser tuners utilizing gratings typically have a tuning axis and a non-tuning axis. Tuning is accomplished either by rotating a grating about the tuning axis directly in the path of the beam or by rotating a mirror against a fixed grating. The subject of this invention and disclosure is the case where the tuning element is a grating directly mounted on a rotational shaft. For proper tuning, the beam or cavity being tuned must strike the grating with the same accuracy in both the tuning and non-tuning directions or axes. The stability and accuracy of the tuning axis positioning is determined by the corresponding accuracy of the device or system rotating the tuning element about the tuning axis and is not a subject of this disclosure.
As the tuning element is rotated about the tuning axis, the beam must maintain perpendicularity with the grating lines. Any departure from said perpendicularity represents motion about the non-tuning axis and therefore an error in tuning. A grating behaves about the non-tuning axis as a simple mirror. No errors will occur if the grating rotational axis, including bearing or flexure translations, run outs and tilts, is parallel to the grating lines. With practical and economical machining, significant and typically excessive errors in the non-tuning axis will occur.
In the prior art, a number of approaches to correct this error have been used with varying degrees of success. At very low speeds, straightforward dual axis adjustment schemes are effective. These methods use fine threads, balls and grooves to iteratively adjust the rotational axis to the grating line parallel. For these techniques, alignment is a time consuming process and attempts to increase tuning rate are hampered by excessive moment of inertia, difficult balance, drive complexities, vibration injection, backlash and general lack of robustness.
McNeil et al in U.S. Pat. No. 4,815,820 proposes a one-axis adjustment scheme. While only one adjustment on the rotating mechanism is required, the approach as disclosed is still mechanically large and complex. The alignment process is improved but is still an iterative one. While the scheme is more agile, its response time is measured in seconds rather than milliseconds and it suffers the same difficulties noted above when increasing tuning rate.
Other, more efficient dual axis approaches, use simpler and therefore usually lighter configurations with bending types of adjustment. Tuning rate, balance and robustness are enhanced significantly. Contrary to the assertion in the referenced patent that dual adjustment schemes are iterative and require knowledge of the value of the two components of error, non-iterative dual axis adjustment schemes can be configured. Agreeably, the hardware for two adjustments oh the rotating axis is undesirable.
The above schemes must deal with the tradeoff between stiffness for stability and the loss of adjustment control due to static friction. All perform poorly when trying to adjust with robustness on a rotational axis down to angles of a few xcexcradians. Further, material incompatibilities, thermal conductivities, thermal expansion coefficients and other issues related to the tuning of high speed and high power lasers place difficult requirements on grating blank material. Grating blanks of ceramic like or crystalline structure may in some cases be preferable or required. These materials do not always machine well and, in some cases, may not be tapped or drilled. This often prevents the integration of the mechanical adjustment with the grating, thereby increasing complexity.
There is therefore a critical need, heretofore unsatisfied, for a structure and, alignment methodology for producing a simple, low moment, balanced and easily aligned rotary grating with both mechanical and thermal robustness.
Preferred embodiments of a directly pivotable grating for agile laser tuners according to the present invention preferably include a rotary shaft supported by bearings or flexures with a machined adjustment flat or plane, a mounting means for receiving a removable grating adjuster, a grating blank with ruled grating lines on a front face and an adjustment plane on a back face for receiving the shaft at its adjustment plane and a bonding means for attaching the grating to the shaft. The grating is preferably fabricated as one in a matrix of gratings and maintains a constant optical and mounting cross-section for efficient machining and ruling. Virtually any grating blank material can be used that is compatible with the lasers optical, thermal, mechanical and dissimilar materials requirements. A one-axis adjustment mechanism preloads the grating onto the shaft""s adjustment plane and performs the alignment under the most favorable condition of low friction before bonding and removal. Filled bonding materials with high thermal conductivities and high shear modulus are available with suitable pre-cure times for alignment. The resulting assembly has high stiffness and, depending on the blank material itself, low thermal gradients even in very high power applications. Because of the inherent simple nature of the components, from a machining point of view, very little movement during adjustment is required meaning that in most cases balance-by-design is adequate.
It is a primary objective of the present invention to provide a simple low moment rotary grating suitable for wavelength tuning of high power agile lasers.
It is another objective to enable the use of lightweight grating blank materials, which cannot be drilled or tapped.
It is another objective to simplify grating blank machining and ruling operations.
It is another objective to improve thermal conductivity, reduce thermal drops and increase laser power capability.
It is another objective to eliminate the grating adjustment mechanism from the rotary assembly.
It is another objective to enable simplified grating alignment under low friction conditions while providing a robust final grating assembly.
It is another objective to enable the use of a wider range of materials for optical elements in a wavelength tuner.